Dehydration of gaseous streams



Dec. 31, 1957 s. A. WILSON DEHYDRATION 0F GASEOUS STREAMS Filed NOV. 12,1955 IN VEN 7'01? 5 ll. Wilson 5 fitter/Icy Unite St tes PatentDEHYDRATION 0F GASEOUS STREAMS Samuel A. Wilson, Tulsa, Okla.

Application November 12, 1953, Serial No. 391,511

1 Claim. (Cl. 260-676) This invention relates to improvements in themethod of dehydrating natural gas streams, and more particularly, is animprovement in dehydrating natural gas in the general manner such asdisclosed in United States Letters Patent No. 2,528,028, issued toArthur F. Barry on October 31, 1950. This application is a continuationin part of applicants co-pending application, Serial No- 221,802, filedApril 19, 1951, and now abandoned.

The above mentioned patent to Barry discloses a method of dehydratingnatural gas wherein the entire gas stream, consisting of both liquid andvapor components, is initially passed through a heat exchanger. The gasstream is then directed through a knock out unit for the removal of freewater from the gas stream. Subsequently, the remaining vapor andhydrocarbon condensates are passed through a liquid bath and thenexpanded to form hydrates and condense a portion of the hydrocarbonvapors. The hydrates and condensates are precipitated into the liquidbath, and the dehydrated gas is utilized to cool the heat exchangerprior to being transferred to a natural gas plant or the like. Theliquid bath must be retained above hydrate formation temperature to meltthe precipitated hydrates.

It is readily seen that when the gas stream is cooled previous to thepassage thereof through the liquid bath, the heat transfer from the gasstream to the liquid bath will be minimized, usually necessitating anauxiliary heat supply for the liquid bath. Furthermore, cooling of thegas stream previous to the removal of the free water minimizes thereduction of the ultimate dew point of the gas, because the saturatedgas cannot be cooled an appreciable degree without forming hydrates.

Another method of dehydration disclosed in the above mentioned patentconsists of initially passing the gas stream through the liquid bath andremoving the free water from the stream. Subsequently, the remainingstream is expanded as set forth above. In the event the incoming gasstream is at a relatively low pressure and temperature, a sufficientheat transfer is not obtained to retain the liquid bath at the desiredtemperature, thereby necessitating an auxiliary heat supply for theliquid bath.

The present invention contemplates an improvement of the methods of gasdehydration disclosed in patent No. 2,528,028, wherein the incoming gasstream from the well is initially passed through a liquid bath to assistin retaining the liquid bath at a temperature about hydrate formation.Subsequently the free Water and hydrocarbon condensates are removed fromthe stream and returned to the liquid bath to assure retention of thebath at a temperature about that of hydrate formation temperature. Theremaining vapor components of the stream are then cooled through heatexchange to substantially hydrate formation temperature by returningdehydrated gas pro duced through use of the process. The cooled vaporcomponents of the stream are subsequently expanded in a manner similarto the above mentioned patent. In returning the free water andhydrocarbon condensates to H Patented Dec. 31, 1957 2 the liquid bath,the bath will be retained at the desired temperature above hydrateformation temperature. By cooling only the vapor components of the gasstream, a greater cooling can be provided with a resulting lower dewpoint (the saturation temperature of a gas at a specified pressure) forthe dehydrated gas.

In addition, the present invention contemplates an improvement overapplicants co-pending application No. 221,802 wherein the lighter gasconstituents of the stream discharged through the reduction valve thatare not condensed are specially cooled prior to any discharge from thesecondary stage separation vessel, so that they may be effectively usedin a subsequent heat exchange function to cool the incoming gas streamprior to discharging into the secondary separating vessel back at atemperature slightly below hydrate formation temperature.

An important object of this invention is to provide an improved methodof dehydrating natural gas streams and particularly natural gas streamsof relatively low temperature.

Another object of this invention is to provide an improved method ofdehydrating natural gas streams utilizing only the heat of the gas toprovide the necessary heat transfer.

Another important object of this invention is to provide an improvedmethod of dehydrating natural gas streams wherein the lighterconstituents of the gas which are reduced and expanded are passed overthe cold zone of the separating vessel without being subjected to thevapors of the hot liquid bath in order to prevent any rise intemperature thereof.

Another object of this invention is to provide an improved method ofdehydrating natural gas streams wherein the free condensates in thestream are removed prior to cooling of the stream, thereby providing alow dew point for the dehydrated gas. 7

A further object of this inventionis to provide an efficient andeconomical method of dehydrating natural gas streams.

Other objects and advantages of this invention will be evident from thefollowing detailed description, read in conjunction with theaccompanying drawings, which illustrate my invention.

In the drawings:

The figure is a diagrammatic view of the necessary apparatus forpractising the improved method of gas dehydration.

Referring to the drawings in detail, reference character 2 designates asuitable flow line leading from a high pres-.

sure distillate well for the transportation of natural gas therefrom.This gas consists of a mixture of hydrocarbon and water in both theliquid and vapor phases, and is usually at a relatively high temperatureand pressure condition, although the condition of the gas will vary inthe various localities.

The gas stream flowing through the conduit 2 is initially passed througha coil 4 disposed in the lower portion of a horizontally disposed vessel6. The vessel 6 comprises,

the second stage separator or dehydration tank of the apparatus utilizedin practising my invention. A liquid bath 8 consisting of a mixture ofwater and hydrocarbon condensates or distillate is maintained in thevessel 6 at a predetermined level 10. The coil 4 is preferablycompletely immersed in the liquid bath 8 to provide a transfer of heatfrom the gas stream to the bath 8 and facilitate the maintenance of thebath above hydrate formation temperature corresponding to the pressurecondition ex isting in the vessel 6. The temperature of the gas streamis only slightly reduced by the heat transfer to the bath 8;

therefore, the temperature of the gas stream remains above hydrateformation temperature as the gas is dis-.

charged from the coil 4. However, the amount of free water in the gasstream is increased for purposes as will be hereinafter set forth.

A conduit 12 conveys the gas discharging from the coil 4 into the upperportion of another horizontally disposed vessel 14. The vessel 14comprises the first stage separator or knock out unit and is provided toremove the free water and liquid hydrocarbon components from the gasstream. As the gas stream enters the upper portion of the vessel 14, thefree water and liquid hydrocarbon components, being heavier than thegas, fall by gravity into the lower portion of the vessel 14 andaccumulate to form a mixture 16. The mixture 16 is periodicallywithdrawn from the vessel 14 through a conduit 13 and directed into theliquid bath 3 provided in the vessel 6. It will be apparent that themixture 16 will be at a relatively high temperature, correspondingessentially to the initial temperature of the gas stream. Therefore, thebath 8 may be conveniently maintained at a temperature well abovehydrate formation temperature by the injection of the mixture 16therein. A suitable control valve 20 is interposed in the conduit 18 tocontrol the flow of the mixture 16 into the vessel 6 and hence thetemperature of the liquid bath 8.

The vapor components of the gas stream, remaining after the removal ofthe free water and liquid hydrocarbon components, are discharged fromthe upper portion of the vessel 14 through a conduit 22 into the tubeside (not shown) of a heat exchanger unit 24. The gas stream is cooledduring passage through the heat exchanger 2 t by the ultimatelydehydrated gas, as will be hereinafter set forth, to substantiallyhydrate formation temperature. In this manner, the ultimate dew point ofthe gas stream may be reduced to a minimum as will hereinafter appear.It has been found in actual practice that the gas stream can be slightlysuper-cooled during passage through the exchanger 24, Without materiallyafiecting the efi'iciency of the heat exchanger 24 or subsequent flow ofthe gas stream.

The cooled gas stream is conveyed from the heat exchanger 24 through aconduit 26 into one end 28 of the vessel 6 above the level of the liquidbath 8. A suitable pressure reducing valve 36) is interposed in theconduit 26 adjacent the vessel 6. The valve 30 provides for an expansionof the gas stream, resulting in a drastic pressure reduction and asubstantial temperature reduction thereof. Hydrates are thereby formed,and a portion of the hydrocarbon components of the gas stream arecondensed. The hydrates, condensates and vapors are discharged into theupper portion of the vessel 6,

whereupon the hydrates and condensates, being heavier than air, areprecipitated downwardly by gravity into the liquid bath 8. The hydratesare melted by the relatively hot liquid bath 8 upon contact therewith tofacilitate their subsequent removal from the vessel 6.

The present invention contemplates a specific improvement over priorsimilar processes relating to gas dehydration in that after the gas ispassed through the reduction valve 30 and expanded in the secondaryseparation vessel 6, a certain portion of this gas is formed intohydrates which are melted in the liquid bath 3, however, the lighterconstituents of the expanded gas that are not condensed are brought intocontact with a vertical baffie member 31 disposed longitudinally at apoint beyond the media point of the vessel 6 looking from the end 28 ofthe vessel at which is disposed the valve 30. The baffie member 31assures that there is a reverse flow of the lighter dry gas vapors andupon such a reverse How, these gases are permitted to flow through adischarge line 32 extending longitudinally in the upper portion of thetank 6 and in spaced relation to the liquid level 10 of the liquid bath8 in order. that these dehydrated lighter portions are cooled or remainin a substantially cooler state to be utilized in the heat exchanger 24as will be hereinafter set forth. The discharge line 32 prevents the hotvapors rising from the liquid bath 8 from contacting the lighterdehydrated gases, and the avoidance of contact with the hot vapors ofthe bath 8 as well as maintaining these lighter constituents at a lowertemperature, substantially to that of the pre-cooling temperature in theupper cold zone of the vessel 6, consequently providing a much moreetfective heat exchange for cooling the oncoming gas stream dischargingfrom conduit 22 into the heat exchanger 24. As has been previouslymentioned, this temperature is maintained slightly below hydrationtemperature before discharging through the conduit 26 and through thepressure reduction valve 39 and expanded in the vessel 6. The effectivemaintenance of this predetermined temperature by utilizing the lighterdehydrated gas for discharge through the conduit 32 back to the heatexchange is a substantial impovement over the process disclosed inapplicants prior application, or that of the Barry patent previouslymentioned.

The distillate is removed from the vessel 6 through an outlet conduit 34which extends upwardly in the vessel adjacent an endv 36 thereof inorder to provide stratification of the water and distillate in thevessel 6. Distillate flowing from the conduit 34 is transferred to anysuitable storage means (not shown) through a suitable control valve 38interposed in the conduit 34 for controlling the flow and maintainingthe level 1! of the bath 8 at a desired height. Water being heavier thandistillate will settle in a suitable water leg or sump as provided onthe lower side of the vessel 6 and the Water is withdrawn when desiredthrough a drain line 42 which in turn is controlled through a suitablevalve 44.

It will thus be apparent that the dry gas vapors, after expansion of thegas stream, remain in the upper cool zone of the vessel 6 and aredischarged through the conduit 32. The flowing of the gas stream fromthe conduit 26 and end 28 of the vessel 6 is through substantially theentire length of the liquid level 10, thereby assuring completeprecipitation of all hydrates formed upon expansion. The dehydrated gaswill obviously be at a lower temperature and is directed into the outletconduit 32 in the upper cooling zone of the vessel and hence to the heatexchanger 24 in order to cool 'by heat exchange the gas stream flowingthrough the tube side thereof as previously set forth. A by-pass (notshown) may be provided in the conduit 32 around the heat exchanger 24 tocontrol the flow of dehydrated gas through the heat exchanger andthereby control the temperature of gas stream discharged into conduit26. A conduit 48 conveys the dehydrated gas from the heat exchanger 24to a suitable storage or natural gasoline plant or the like (not shown).

In summation, the gas stream is initially passed through a liquid bathcontained in a second stage separator to facilitate maintenance of thebath above hydrate formation condition and to increase the free watercontent of the gas stream. The free water and liquid hydrocarboncomponents are then removed from the gas stream in the first stageseparator and returned to the liquid bath to assure the retention of thetemperature of the bath at the desired degree. Therefore, thetemperature of the the liquid bath may be controlled as desired andwithout the necessity of an auxiliary heat supply, even though theinitial temperature of the gas stream is relatively low.

The gas stream remaining after the removal of the free water and liquidhydrocarbon condensates is cooled to substantially hydrate formationtemperature by the ultimately dehydrated gas. The cooled gas is thenexpanded to reduce the pressure and temperature thereof and formhydrates as Well as condense a portion of the remaining hydrocarboncomponents.

-t will be readily seen that the maintaining of a lowtemperature for thedehydrated. and the lighter constituents of the gas vapors-by dischargethrough the outlet conduit 32 will provide a lower temperature for heatexchange, thereby maintaining a greater cooling of the gas streamimmediately prior to expansion which in turn will provide a greaterdehydration.

From the foregoing, it is apparent that the present invention providesan improved method of dehydrating natural gas, particularly adapted forthe dehydration of gas streams having a relatively low temperature. Thefree water and liquid hydrocarbon components of the gas stream areutilized to melt hydrates intentionally formed by expansion of the gasstream, thereby negativing the necessity of an auxiliary heat supply.The non-dehydrated lighter constituents of the gas are cooled subsequentto the expansion of the gas stream, thereby maintaining a maximum lowtemperature on the gas which is discharged for return to the heatexchange in order to maintain a temperature slighly below hydrationformation temperature prior to expansion and thus obtaining a maximumultimate dehydration of the gas stream. A gas stream may therefore bedehydrated to a greater extent than has formerly been possible and theinherent properties of the gas are utilized the maximum degree. it isalso apparent that the present invention provides an economical methodof dehydrating natural gas existing at a high temperature and pressure.

It will thus be apparent that the present process provides a cooler gasin heat exchange thereby reducing the volume of heat exchange requiredand is particularly efiective on gas streams produced in hot wells.

The free water and liquid hydrocarbon components that were knocked outin the primary separation chamber 14, and hence discharged through theconduit 1% to the liquid bath 8 may be lay-passed through a suitableconduit (not shown) for entry into an inlet (not shown) in proximity ofthe end 36 of the separator 6, in order to come in contact with a weirtype baffle member 49 thereby directing the free water from conduit 18into the opposite end 36 of the vessel 6 as distinguished from the end28. The by-passing of the knock-out liquids is preferably usually donein wells having a high parafiin content in the influent mixture in orderto discharge it against the weir type baffle 49 disposed at the coolerend of the vessel 6, rather than discharging it into contact with thehot liquid bath 8 which would have a tendency to melt the paraffin andcause deposit of the parafin against the coils 4 disposed in the bath 8.

Changes may be made in the combination and arrangement of parts asheretofore set forth in the specification and shown in the drawings, itbeing understood that any modification in the precise embodiment of theinvention may be made within the scope of the following claim withoutdeparting from the spirit of the invention.

1 claim:

A method of dehydrating a natural gas stream, consisting of passing thegas stream through a liquid bath to trans fer a portion of the heat ofthe stream to the liquid bath, removing the free water and hydrocarboncondensates from the stream, cooling the remaining vapor components ofthe stream to substantially hydrate formation temperature, expanding thecooled gas to form hydrates, precipitating the hydrates into the liquidbath, directing the separated free Water and hydrocarbon componentsdirectly into the liquid bath to retain the liquid bath above hydrateformation temperature for melting the hydrates, separating the water andhydrocarbon components of the liquid bath for separate removal,utilizing the dehydrated gas to cool the vapor components of the gasstream, directing the dehydrated gas through the cooling stage of theinfiuent vapor components in a reverse direction thereto andsimultaneously preventing contact therewith of the hot gases from theliquid bath in order to maintain the dehydrated gas at a temperaturesufficient to cool the incoming vapor components of the stream.

References Cited in the file of this patent UNITED STATES PATENTS2,356,407 Hutchinson Aug. 22, 1944 2,399,723 Crowther May 7, 19462,528,028 Barry Oct. 31, 1950 2,538,947 Ragatz Jan. 23, 1951 2,665,565Parks Jan. 12, 1954

